Temperature probe



Aug. 8, 1967 N. A. FORBES TEMPERATURE PROBE 3 Sheets-Sheet 2 OriginalFiled Nov. 19, 1964 EKUI...

ZUn. JOEFZOU m $tma 1 l I l i ll um Jomhzou mow hww m U521: 0

mmN

, Aug. 8, 1967 N. A. FORBES TEMPERATURE PROBE 3 Sheets-$heet OriginalFiled Nov. 19, 1964 FIG.3

WF IO MUZ PQWMI 180 200 TEM P. F

United States Patent Ofiflce 3,335,382 TEMPERATURE PROBE Norman A.Forbes, Louisville, Ky., assignor to American Radiator & StandardCorporation, New York, N.Y., a corporation of Delaware Originalapplication Nov. 19, 1964, Ser. No. 412,482, now Patent No. 3,277,946,dated Oct. 11, 1966. Divided and this application June 20, 1966, Ser.No. 568,090

1 Claim. (Cl. 338-22) ABSTRACT OF THE DISCLOSURE A thermistor probe ispositioned within the boiler to monitor the fluid temperature of a highheat flux boiler system. The system also includes a flame sensor tosense for the burning of fuel. Signals from the thermistor probe, theflame sensor and an external thermostat interact to regulate a fuelvalve and a blower for controlling the feeding of fuel to a burner whichheats the boiler fluid.

This is a division of application Ser. No. 412,482 filed Nov. 19, 1964.

This invention relates to heater control systems and more particularlyto fast operating safety control systems employing temperature limitcontrols and flame controls.

In presently available oil burning and gas burning heaters, safetycontrols are included to limit the temperature of the boiler and toprevent the prolonged release of unburned fuel because of faultyignition. Therefore, presently available heaters usually includetemperature limit controls employing liquid filled bulbs or bimetallicelements which sense temperature and when the temperature exceeds agiven value turn off the fuel supply. Such temperature sensors have atime constant in the order of ninety seconds. In addition, suchpresently available systems include combustion controls to releaseunburned fuel for a trial-for-ignition" time at the start of eachcombustion cycle. If combustion is not obtained Within a given time, thefuel feed is turned off and the controls must generally be manuallyreset. Thus, the combustion controls prevent the sudden ignition oflarge quantities of fuel which may result in a damaging explosion.

Lately, there has become available a high heat flux boiler which has alow water content, a low combustion area, and a low mass, all relativeto the rate at which fuel is burned. Accordingly, if heat removal fromthe boiler stops because of some malfunctioning, the temperature risesat a rate much faster than with boilers in heretofore available heatingsystems. For example, the rate of rise of temperature within a high heatflux boiler can be of the order of five degrees Fahrenheit per second.Thus, if the temperature limit control is to lag the boiler watertemperature by no more than ten degrees, the control must have a thermaltime constant of about two seconds. Therefore, it is apparent thatconventional temperature limit controls with time constants forty tofifty times longer cannot be employed with high heat flux boilers.

Furthemore, since high heat flux boilers use a forced draft rather thanan induced draft, they can store more unburned fuel than conventionalboilers if the blower fails. Accordingly, the trial-for-i-gnition timemust be kept as short as possible to minimize the accumulation ofunburnt fuel which may be detonated. However, the

3,335,382 Patented Aug. 8, 1967 trial-for-ignition time must be longenough for a flame control to recognize a flame. Therefore, if thetrial-forignition time must be short for safety reasons, the flamecontrol must be fast acting.

It is, accordingly, a general object of the invention to provide animproved system forcontrolling the flow of for igniting the fuel fed tothe burner sensing apparatus.

It is yet a further object of the invention to provide improved flamesensing apparatus which responds to the presence of flame in arelatively short time.

It is a still further object of the invention to satisfy the aboveobjects with control apparatus that is failsafe.

Briefly, the invention contemplates control apparatus for a heatingsystem which includes a fuel burner, a boiler containing fluid as a heatexchange medium, fuel feeding means and fuel igniting means. The controlapparatus comprises a temperature limit sensor including a thermistor inintimate thermal contact with the boiler fluid to prevent operation ofthe fuel feeding and igniting means when the fl'uid temperature exceedsa given value, and a flame sensor means to terminate operation of thefuel feeding means if a flame is not sensed within a given time afterstart of operation of the fuel feeding and fuel igniting means, or if aflame fails during normal operation.

Some features of the invention are concerned with insuring that if theelements of the control means responsive to the temperature sensor orthe flame sensor as well as the temperature sensor and flame sensorthemselves fail, the system shuts down with the fuel feeding means andthe fuel igniting means deenergized.

Another feature of the invention concerns a fast operating temperatureprobe for sensing the temperature of the fluid in the boiler of aheating system.

Other objects, features and advantages of the invention will be apparentfrom the following detailed description when read with the accompanyingdrawing which shows, by way of example and not limitation, the nowpreferred embodiment of the invention.

In the drawing:

FIG. 1 is a block diagram of a heating system incorporating controlapparatus according to the invention;

FIG. 2 is an electric-a1 schematic diagram of the control apparatusemployed in the heating system of FIG. 1;

FIG. 3 is a side view partially in section of a temperature probe havinga thermistor in accordance with a feature of the invention; and

FIG.-4 is a graph of the resistance in ohms as a function of temperaturein degrees Fahrenheit of the thermistor of FIG. 3.

Referring now to FIG. 1, a heating system 10 is shown comprising aburner BU surrounded by a boiler BO containing a fluid such as waterconnected via conduits 12 and 14 and circulator CI to a heat sink HSwhich may be radiators or the like for heating the rooms of a building.Burner =BU may be a high heat flux burner as described in the c-opendingapplication Ser. No. 397,804 filed Sept. 21., 1964, by Wilson J. Witten,Jr. and assigned to the same asseignee. Boiler BO may be of the typedescribed in the copending application Ser. No. 397,747 filed Sept. 21,1964, by Wilson J. Witten, Jr. and assigned to the same assignee.

In any event, fuel such as a gaseous hydrocarbon from a gas source GS iscontrollably fed via a solenoid operated gas valve GV to a blower BLwhere it is mixed with air and force fed to burner BU. An ignition meansIG which may employ a spark plug 16 and an ignition coil is positionednear burner BU to ignite the gaseous mixture diffusing therefrom. Aflame sensor PC which preferably includes a photoconductive element and,if necessary, a lens 18 is positioned near burner BU for sensing theburning of fuel. Fitted into boiler B0, in intimate contact with thefluid therein, is a temperature probe 20 which is included intemperature sensor means TS. A control circuit CC energized bythermostat TH, positioned in a region to be heated, controls theoperation of blower BL, ignition means IG and gas valve GV in accordancewith information received from temperature sensor TS and flame sensorPC.

Generally, when the temperature of heat sink HS falls below a particularvalue, thermostat TH signals control circuit CC of this fact via cable22. Assuming normal (fault-free) operation, control circuit CC energizesblower BL via cable 24 and, a short time thereafter, gas valve GV viacable 26 and ignition means IG via cable 28.

It should be noted that gas valve GV and ignition IG are energized agiven period of time after the energizing of blower BL for permittingthe pre-purging of the firing portion of the system to protect against aslowly leaking gas valve.

If there is a leak in the gas valve, a combustible mixture of gas andair may accumulate. Therefore, a blower BL will blow this potentiallydangerous mixture out the flue before ignition is attempted. If astanding pilot is employed instead of an intermittent ignition means anyslowly leaking gas is burned and does not accumulate and a pre-purge isnot necessary.

In any event, when ignition means IG ignites the fuel, flame sensor PCdetects this fact and signals control circuit CC via cable 30. Controlcircuit CC discontinues energizing ignition means IG and the flame ofburner BU sustains the burning of fuel. However, if flame sensor PC doesnot indicate the sensing of a flame within a given period of time,control circuit CC stops energizing gas valve GV and ignition means IG,and may give an indication of this fact. During operation of the system,whenever the temperature of the fluid in boiler BO exceeds a givenvalue, this fact is transmitted via cable 32 to control circuit CC whichdeenergizes blower BL, gas valve GV and ignition means IG and preventstheir reenergization until the fluid temperature drops below the givenvalue. p

The control apparatus shown in FIG. 2 will now be described in detail.It should be noted that throughout the description relays and theirassociated coils and contacts will be mentioned. Generally, the relaywill be specified by its winding reference character, for example, therelay 1R has a winding 1R. In addition, the contact sets of the relaysare designated by a reference character comprising the referencecharacter of the relay followed by a number. For example, relay 1R has acontact set 1R1 and a contact set 1R2. All the relays are shown in theirunenergized state.

A source of 120 volt, 60 cycle alternating current feeds busses 40 and42. Dis-posed across busses 40 and 42 is a circulator CI for pumpingboiler fluid around the heat exchange circuit comprising the boiler Band the heat sink HS.

Bus 40 is connected to bus 43 by means of contact set 4 3K1 of relay 3K,which is part of the temperature limit control TLC, to be describedlater.

Gas valve GV, blower BL and ignition means IG are energized from busses43 and 42 under control of the contact set 1R1 of relay 1R(a firstcontrol means). When contact set 1R1 closes, alternating current isapplied to blower via leads 24A and 24B of cable 24 and to the windingof time delay relay 2K to start the pre-purge phase of a combustioncycle. Blower BL blows any accumulated gases out the flue. After a givenperiod of time, relay 2K energizes closing contact sets 2K1 and 2K2. Thelatter closes a circuit between the alternating current busses 43 and 42via leads 26A and 26B of cable 26 and the solenoid of gas valve GV. Gasvalve GV opens causing gas to enter blower BL. Contact set 2K1 closes acircuit between busses 43 and 42 via leads 28A and 28B and ignitionmeans -IG which generates a spark as long as energized. Thetest-for-ignition phase of a combusition cycle begins. Du-ring normaloperation, i.e., no malfunctioning, after a short period of time contactset T1 of a relay T opens interrupting the alternating current toignition means IG. The spark terminates but fuel continues to burnbecause gas valve GV and blower BL are still energized and will remainso until contact set 1R1 opens. It should therefore be apparent that bycontrolling the operation of relay 1R it is possible to control theoperation of the fuel feed and the fuel igniting.

The control for primary control PC will now be described. Transformer2TR has its primary winding 2TRP connected to busses 43 and 42 and itssecondary winding 2TRS connected to. busses 44 and 50. The winding ofrelay IR is in the series circuit comprising bus 44, contact set SS1,bus 46, thermostat TH, contact set 1K1, relay network 48 and bus 50. Thewinding of relay 2R is in the series circuit comprising bus 44, contactset SS1, bus 46,

diodes D6, D7, D8 and D9, bus 58, flame control FC, bus 60, and bus 50.

During normal operation, contact set 1K1 of relay 1K is closed because,as will hereinafter become apparent, relay 1K which is part of thetemperature limit control TLC will be energized as long as the fluid inboiler B0 is below a given temperature. Accordingly, when thermostat THwhich may include a bimetallic switch closes to demand more heat, relayIR is energized. Note a current path is established through relaynetwork 48 as follows: the winding of relay SS, contact set T2, lead '52and contact set 2R2. Therefore, it is seen that relay 1K can control theenergization of relay 1R which in turn energizes blower BL, gas valve GVand ignition means IG.

'Relay 1K is part of temperature limit control TLC (a temperaturesensitive control means). Also included in temperature limit control TLCis a source of alternating current which includes: transformer lTR whoseprimary winding lTRP is connected to busses 40 and 42; a source ofdirect current comprising semi-conductor diode D1 and capacitor C1; afusible element F1 (circuit breaker) connecting one arm of secondarywinding ITRS to the anode of diode D1; a thermistor'TS which isconnected between one end of the winding of relay 1K and ground; a lead54 connecting the junction (output terminal of source of direct current)of diode D1 and capacitor C1; a lead 56 connecting the junction (outputterminal of source of dire-ct current) of an end of secondary windinglTRS and capacitor C1; four diodes D10, D11, D12 and D13; and relay 3K,connected across transformer secondary lTRS by contact set 1K1 (thepurpose of diodes D10, D11, D12 and D13 is to form a contact suppressorto protect contact set 1K1 from the effects of interrupting current tocoil of relay 3K, which is inductive). As will hereinafter becomeapparent, thermistor TS is in intimate thermal contact with boilerfluid. Thermistor TS is a temperature sensitive solid state device whoseresistance varies with temperature (see FIG. 4). In particular,thermistor TS is designed with a positive temperature coeflicien-t sothat its resistance increases markedly as its temperature exceeds 180degrees Fahrenheit and preferably at least at the rate of five percentper degree Fahrenheit in the range between 190 and 230 degreesFahrenheit. The temperature range specified is for when water is theboiler fluid. Relay 1K is a relay having reed contacts whose elasticforces bias the con-tact set 1K1 to the open position. Therefore, relay1K when not energized automatically assumes the open state.

Normally, if the temperature of the boiler fluid is below about 220degrees Fahrenheit the resistance of thermistor TS is such thatsufficient current flows through the winding of relay 1K so that contactset 1K1 closes. However, at higher boiler fluid temperatures,, theresistance of thermistor TS is high enough to limit the current throughthe winding of relay 1K to a value insufficient to hold contact set 1K1closed. Therefore, relay 3K is deenergized, and contact set 3K1 opens,deenergizing transformer 2TR and the entire primary control PC, thesolenoid gas valve GV, the blower BL, and the ignition means IG.Therefore, temperature limit control TLC in response to the temperatureof boiler fluid, and through the agency of a temperature sensitiveresistance, controls the energization of relay 1R by means of contactset 3K1 which is required to complete the electrical circuit between theprimary winding ZTRP and bus 40. Interrupting the flow of gas when theboiler water tends to overheat creates a condition in which thethermostat is calling for heat and there is no flame. To prevent thesafety switch SS from interpreting these conditions as an ignitionfailure and operating, the temperature limit circuit TLC switches offthe entire primary control PC, so that safety switch SS is not energizedand therefore cannot operate.

Assume temperature limit control TLC is working normally. Then whenthermostat TH demands more heat, relay IR is energized initiating thepre-purge phase of the combustion cycle which is followed by thetest-for ignition phase as previously described. By virtue of the relaynetwork 48 the continued energization of relay 1R becomes dependent onthe mode of operation of relay 2R and relay SS. Relay SS is a time delaycontrol means in the form of a time delay relay having a normally closedcontact set SS1. If the winding of relay SS continuously receivescurrent for greater than a given period of time (actually the normaltime for the fuel in burner BU to ignite) contact set SS1 opens,interrupting the circuit from transformer secondary 2TRS to bus 46.Therefore, current stops flowing through the winding of relay IR andcontact set 1R1 opens shutting down the system. Relay SS isadvantageously provided with a manual reset to reclose the contact setSS1 so that human intervention is required in such a case. It should benoted that contact set 2R1 shunts the winding of relay SS so that whenrelay 2R is energized the current initially flowing through relaywinding SS is diverted therearound. Accordingly, if relay 2R isenergized before the end of the period of time required to energizerelay SS, the latter is not energized.

Relay 2R is part of an electrical resistance sensitive control meanswhich includes a bridge rectifier comprising semi-conductor diodes D6,D7, D8 and D9. The junction of the anodes of diodes D6 and D8 areconnected to one end of the winding of relay 2R whose other end isconnected to the junction of the cathodes of diodes D7 and D9. Thejunction of the anode of diode D7 and the cathode of diode D6 isconnected to bus 46. The junction of the cathode of diode D8 and theanode of diode D9 is connected via flame control FC (an electronicrelay) to bus 50. The resistance of flame control PC has a valuesufficiently high to prevent energization of relay 2R if flame sensor PCdoes not sense a flame on burner BU. When flame sensor PC senses a flamethe resistance of the flame control FC decreases to such a value thatrelay 2R is energized.

Flame control FC includes a bridge rectifier comprising semi-conductordiodes D2, D3, D4 and D which I receive alternating current via leads 58and 60. Lead 58 connects the junction of diodes D8 and D9 to thejunction of the anode of diode D3 and the cathode of diode D2. Lead 60connects the junction of the cathode of diode D4 and the anode of diodeD5 to bus 50. Silicon controlled rectifier SCR has its cathode connectedto the junction of the anodes of diodes D2 and D4 and its anodeconnected to the junction of the cathodes of diodes D3 and D5. Aresistor R1 is connected between the control input and the anode ofsilicon controlled rectifier SCR. Flame sensor PC, a photoconductiveelement made of cadmium sulfide, for example, is connected between thecontrol input and the anode of silicon controlled rectifier SCR. Siliconcontrolled rectifier SCR is a solid-state switch which closes when agiven current flows into its control input when a positive voltage ispresent between its anode and cathode, and opens whenever the currentinto its control input is removed and the anode-cathode voltage falls tozero. It can be considered to be a solid-state thyratron. Resistor R1and flame sensor PC control the amount of current fed into the controlinput. When flame sensor PC does not detect a flame its resistance ishigh and when it detects a flame its resistance is low. When theresistance of the flame sensor PC is high the effective resistance offlame control FC is high and when the resistance of the flame sensor PCis low the effective resistance of flame control PC is low. Therefore,the presence or absence of flame is reflected by the value of theresistance of the flame sensor PC which determines the effectiveresistance of flame control PC for controlling the energizaiton of relay2R.

When burner flame is sensed, relay 2R is energized and the alternatingcurrent path through relay network 48 is via contact set TRl, lead 52,contact set 1R2 and the winding of relay T. Relay T is a time delayrelay which energizes after its winding has continuously receivedcurrent for at least a given period of time. When relay T energizes: itscontact set T1 opens deenergizing ignition means IG and the burner flamebecomes self sustaining; and its contact set T2 opens to positivelyprevent the receipt of current by the winding of relay SS. Fuel will nowburn until thermostat TH opens indicating the end of the demand for heator, relay 3K deenergizes indicating boiler fluid is overheating.

The temperature probe 20 will now be described. In FIG. 3 thetemperature probe 20 is shown as screw fitted into an opening in theboiler BO through the agency of threaded fitting 100. Extending from thebottom of fitting is a hollow tube 102. A circular disc 104 is fixed tothe base of tube 102. Disc 104 is made of a good thermal conductor suchas copper or one of its alloys. Alternatively, disc 104 can be made of amaterial, for example, molybdenum, that combines relatively high thermalconductivity with a coeflicient of thermal expansion that matches thatof the thermistor. Thermally conductively fixed to the face of disc 104inside tube 102 is thermistor TS. Therrnistor TS is fixed to disc 104 bymeans of a thin layer of solder so that there is good thermal andelectrical conduction therebetween. Connected to thermistor TS are thesignal leads of cable 32 which are brought out through a central openingin threaded fitting 100 for connection to the control circuitry.

In order that thermistor TS rapidly follows the temperature of theboiler fluid the outer face of disc 104 is in intimate contact with theboiler fluid; disc 104 is made of a good thermal conductor; and themedium for fixing thermistor TS to the inner face of disc 104 is also agood thermal conductor. Furthermore, the hollow tube 102 has a minimumwall thickness which is determined merely by mechanical supportconsiderations and rigidity. Thus, the amount of heat that tube 102 canconduct away from disc 104 to the boiler wall is minimized. In addition,the disc 104 is preferably made of material whose thermal coefiicient ofexpansion closely matches that of the material of the thermistor TS. Inthe usual case, a major limitation on the operating temperature range isthe shear stress developed because of the thermal mismatch and thisstress is larger as the thermistor diameter increases. Therefore, as themismatch in thermal expansion coefficient is reduced, thermistor TS canbe made appreciably larger in diameter and, accordingly have a quickerthermal response. It should be noted that, in principle, one mightexpect the time constant to be kept the same by making the thermistorand solder very thin. In practice, it is expensive to make thethermistor thin, and very difficult to make the solder very thin so thatthe solder and thermistor thicknesses (for reasonablypriced devices)remain fairly constant.

There will now be discussed certain reliability features of the system.

First, the flame sensor PC (FIG. 2) which is a photoconductor isincorporated in the circuit in a particular way so that it has a longstable life by operating at low electrical power levels. When no flameis sensed, the resistance R of the photoconductor is very high and evenif it must withstand the full DC voltage developed by the bridgerectifier of flame control PC the electrical power (P=V /R) is very lowbecause of the high value of the resistance R. When there is flame theresistance of the photoconductor is very low but the silicon controlledrectifier fires early in the alternating-current cycle and the voltageacross the photoconductor is equal to the voltage drop in the siliconcontrolled rectifier which is extremely low. Therefore, the powerdissipated in the flame sensor PC (photoconductor) is again low due tothis low voltage.

Other reliability features are concerned with the failsafe operation ofthe temperature limit control TLC and the flame control PC. Forfail-safe operation it is required that the failure of any component ina unit under consideration either causes the feeding of fuel to stopimmediately or at the beginning of the next combustion cycle.

Consider first the temperature limit control TLC (FIG. 2). If thermistorTS open-circuits, relay 1K either deenergizes or cannot be energized.Therefore, since relay 1K is a reed relay whose reed contacts are selfbiased to open, relay 1R deenergizes or cannot be energized. Whenthermistor TS short-circuits, the excess current drawn melts fusibleelement F2 interrupting the current through the winding of relay 1Kwhich deenergizes. If the coil of relay 1K shortor open-circuits, relay1K deenergizes.

If capacitor C1 open-circuits, pulsating direct current is fed to thecoil of relay 1K causing periodic energization of relay 1K. However, theenergization is for only thirty percent (approximately) of thealternating-current cycle and not enough to cause relay 1K to energize.When capacitor C1 short-circuits, excess current is drawn causingfusible element F1 to open preventing current flow through the windingof relay 1K. If diode D1 open-circuits, no current flows through thecoil of relay 1K. When diode D1 short-circuits, the excessive currentvaporizes fusible element F1 and no current can flow through the windingof relay 1K. If transformer 3TR openor shortcircuits no current isavailable to energize relay 1K. If any one of the diodes D10, D11, D12or D13 openor shortcircuits, relay 3K receives only half wave power;that is, for only half of the alternating current cycle, so that relay3K does not pull in. Any overtemperature condition caused by one of thediodes D10, D11, D12 or D13 short-circuiting is prevented by fuse F1blowing. Hence, it isseen that the failure of any component results inrelay 1K or 3K being in the deenergized state and since the energizationof relay 3K requires that relay 1K be in the energized state relay 3K isforced into the deenergized state when relay 1K is not energized. And,since the energization of gas valve GV occurs only when relay 3K isenergized no fuel is fed as long as relay 1K is in the deenergizedstate.

Now consider the flame control PC. If one of the diodes D2, D3, D4 or D5open-circuits the coil of relay 2R receives current for only half ofeach alternating-current cycle and therefore cannot pull in, though itcan hold in permanently. If one of the diodes D2, D3, D4 or D5short-circuits, relay 2R is continuously pulled in. When siliconcontrolled rectifier SCR open-circuits, relay 2R cannot be energized;and when silicon controlled rectifier SCR short-circuits, relay 2R iscontinuously energized. If resistor R1 open-circuits, the capacitivecurrent coupled to the flame sensor PC from conductors carryingalternating-current is sufficient to cause silicon controlled rectifierSCR to continuously conduct and relay 2R is continuously energized. Whenresistor R1 is short-circuited, the gate cathode junction of the siliconcontrolled rectifier SCR is shorted, and it cannot be fired. When flamesensor PC is open circuited, relay 2R cannot be energized; and whenflame sensor PC is short-circuited, relay 2R is continuously energized.In summary, any component failure in the flame control FC either resultsin the inability to energize relay 2R, or in the continuous energizationof relay 2R. If relay 2R cannot be energized, then, when relay 1R isenergized, at the start of a combustion cycle, alternating-current flowsthrough the relay network 48 via the winding of relay SS, contact setT2, lead 52 and contact set 2R2 and this path never changes. Therefore,relay SS energizes opening contact set SS1 to deenergize relay 1R. Ifrelay 2R is continuously energized, then, at the start of a combustioncycle, no path is closed through the relay network 48 to permit acurrent flow for the winding of relay 1R since contact set 1R2 is open(relay 1R is not yet energized) and contact set 2R2 is open (relay 2R isenergized). The feeding of fuel is dependent on energization of gasvalve GV which depends on contact set 1R1 being closed; and the latteronly closes when relay IR is energized. Therefore, it is seen that thefailure of any component in flame control FC results in the preventionof the feeding of fuel to the burner BU.

There has thus been shown improved control apparatus for the fuelburning portion of a heating system which by employing optical flamesensing and thermistor sensing of boiler fluid temperature permits therapid response to conditions demanding the shut down of the system.While the control apparatus is ideally suited for compact boilers it canbe used effectively with other types. Furthermore, the disclosed controlapparatus by virtue of the novel combination of elements employed is notonly fast acting and reliable but also extremely safe because of itsfailsafe features.

While only one embodiment of the various aspects and features of theinvention has been shown and described in detail there will now beobvious to those skilled in the art many modifications and variationssatisfying many or all of the objects of the invention, but which do notdepart from the spirit thereof as defined in the appended claim.

What is claimed is:

A temperature probe for sensing the temperature in a boiler whichincludes a housing defining a chamber, a fluid in said chamber, and saidhousing being provided with a threaded opening; said probe comprising ahollow threaded metal fitting for meshing with the threads of theopening, a hollow tube of thin conductive material, one end of saidhollow tube being thermally and electrically connected to said fitting,a thin metallic disc which is thermally and electrically conductiveconnected to the other end of said hollow tube with the peripheral edgeof said disc extending radially beyond the entire circumference of saidhollow tube, said disc having a given coeflicient of thermal expansion,a thermistor having a coefficient of thermal expansion substantiallyequal to said given coefficient of thermal expansion, adhesive means forfixing said thermistor in intimate thermal contact with the face of saiddisc which is within the confines of said hollow tube, and electricalsignal conductor means connected to said thermistor and extendingthrough said holcuit.

References Cited UNITED STATES PATENTS Becker 338-22 Medlar 338-30Perkins et al. 338-23 Talbott 73-362 Seney 338-28 Elliot et al. 338-28Sion 338-28 Boddy 338-28 Barton 73-362 Summerer 338-25 Vanik et al.338-25 Matson et a1. 338-22 RICHARD M. WOOD, Primary Examiner. Bennett333 3 10 ANTHONY BARTIS, Examiner.

W. D. BROOKS, Assistant Examiner.

